My invention pertains to a method of, and a system for, optoelectronically measuring quantities of electricity or magnetism by use of two beams of light of different wavelengths. The opto-electronic measuring method and system of this invention have particular utility as under circumstances where electromagnetic disturbances preclude the use of more common electrical measuring instruments or where the use of potential generators involves danger.
The invention deals more specifically with improvements in optoelectronic measuring systems of the type described and claimed in Japanese Patent Application No. 55-22449, filed by the assignee of the instant application. I will incorporate the contents of this prior Japanese application herein insofar as is necessary for a full understanding of my present invention.
Two well known methods exist for optoelectronic measurement of quantities of electricity or magnetism. One takes advantage of the electrooptical Pockels effect for measurement of voltages or electric charges. The other utilizes the magnetooptical Faraday effect for determination of current magnitudes or magnetic field strengths.
For the measurement of voltages or charges by the Pockels effect, light is directed into a Pockels crystal such as potassium dihydrogen phosphate (KDP) or lithium niobate (LiNbO.sub.3). The incident light gives rise to two characteristically polarized components, which propagate through and emerge from the Pockels crystal with a phase difference therebetween proportional to the strength of the electric field being impressed to the crystal. The two polarized beam components are subsequently passed through a polarizing filter, which selectively absorbs components of the incident light. Coming out of the polarizing filter is a beam having an intensity modulated to represent the phase difference between the two incoming waves. One can know the applied voltage or charge from the intensity of this intensity-modulated beam.
The Faraday effect lends itself to use for the measurement of current magnitudes or magnetic field strengths in the following manner. When a beam of plane-polarized light passes through a Faraday medium in the direction of an applied magnetic field, the plane of polarization rotates through an angle proportional to the field strength. Subsequently passed through a polarizing filter, the polarized beam has its Faraday rotation translated into the intensity of the output beam. The measurement of the intensity of this intensity-modulated beam provides an indication of the current magnitude or magnetic field strength.
These optoelectronic measuring schemes based on the Pockels and Faraday effects have gained greater utility with the recent development and refinement of optical fibers capable of acting as "light pipes" or carriers of optical energy.
In a typical conventional electric or magnetic measuring system; optical fibers find use as transmission lines from a monochromatic light source to an electro- or magnetooptical modulator, where the incoming light is intensity modulated either electrically or magnetically, and from this modulator to a photodetector. The light source usually takes the form of a laser or light-emitting diode (LED). For the measurement of voltages or electric charges the modulator may comprise a polarizer, Pockels or like electrooptical crystal, phase element such as a quarter-wave plate, and polarizing filter. For the measurement of current magnitudes or magnetic field strengths, on the other hand, the modulator may comprise a polarizer, Faraday or other magnetooptical medium, and polarizing filter. Coming out of the modulator, the electrically or magnetically intensity-modulated beam passes through the optical fiber into the photodetector as typified by a photodiode or photomultiplier tube. The photodetector derives from the incoming beam a signal indicative of the electric or magnetic quantity being applied to the modulator.
Optical fibers available today have still certain weaknesses, however. Take, for example, the familiar multimode fiber of the type having a central portion, or core, surrounded by an outer layer of material with a slightly lower refractive index, called the cladding. Light rays travel through the fiber by undergoing series of total reflections at the core-cladding interface. Thus are the light rays trapped inside the fiber core. Let n.sub.1 be the refractive index of the core, and n.sub.2 the refractive index of the cladding, n.sub.1 being greater than n.sub.2 as aforesaid. The maximum glancing angle, or critical angle, .theta..sub.c at which total reflection occurs at the core-cladding interface is expressed as EQU .theta..sub.c =cos.sup.-1 n.sub.2 /n.sub.1. (1)
A problem arises because of the almost unavoidable bends and curves of the optical fiber in use. At such bends or curves, part of the traveling rays of light will strike the core-cladding interface at more than the critical angle .theta..sub.c and so, instead of being reflected back into the core, pass into the cladding to be absorbed by the jacket applied to the outside of the fiber. Thus light loss in optical fibers is subject to change depending in part on their linearity, or the degree to which they are bent or curved.
Another possible cause of variation in transmission loss is ambient temperature. In some optical fibers the refractive indexes of the core and the cladding change differently with ambient temperatures. An example of such fibers is the one having a core made of fused quartz and a cladding made of a silicone resin. Then the critical angle .theta..sub.c, and in consequence the light loss, of the fiber vary with ambient temperatures.
Still another possible cause of variable transmission loss in optical fibers is the connectors or couplings for detachably connecting the lengths of fibers end to end. In the mentioned electric or magnetic measuring system the use of such fiber connectors is common at the exit of the light source, at the entrance and exit of the electro- or magnetooptical modulator, and at the entance of the photodetector, for the ease of system installation or transportation. Each fiber connector holds two optical fiber lengths endwise against each other, with a preassigned spacing therebetween, as will be later explained in more detail in connection with one of the drawings attached hereto. The connectors employ cap nuts for the detachable connection of the fiber lengths. The cap nuts are easy to loosen in the use of the measuring system in locations subject to mechanical vibrations. Thereupon the spacing between the opposed ends of the fiber lengths may change, or either or both of the fiber lengths may move out of alignment with each other. Such changes in the end-to-end spacing of the fiber lengths, or their misalignment, no matter how small, incurs very substantial fluctuations in coupling loss because the core diameter of the optical fiber under consideration is as small as 50 micrometers or so.
The foregoing will have made clear that variable light loss occurs almost unavoidably in the optical fibers of the above defined type by reasons of: (1) the bends or curves of the fibers; (2) changes in ambient temperature; and (3) the loosening of the fiber connectors. The fluctuations in light loss due to such unpredictable factors had rendered the noted conventional system quite unreliable, unsuitable for accurate measurement of electric or magnetic quantities.
In the above referenced Japanese Patent Application No. 55-22449, therefore, I proposed a novel optoelectronic measuring system (shown in FIG. 1 of the accompanying drawings) free from the drawbacks of the more conventional system. The measuring system according to this Japanese application uses two light beams of different wavelengths, as will be detailed later. It substantially succeeds in preventing the optoelectronic measurement of electric or magnetic quantities from being affected by the variable light loss due to the listed unpredictable factors.
As has later proved, however, the known two-beam measuring system has a problem in connection with the relative wavelengths of the two light beams. The wavelengths of the beams should be close to each other in order for the beams to suffer approximately the same transmission losses due to the bending or curving of the optical fibers and approximately the same coupling losses due to the loosening of the fiber connectors. The use of beams of such close wavelengths results, however, in the reduced sensitivity of the system with respect to the quantities to be measured. For this reduced sensitivity the prior art system has required expensive equipment for accurate measurement of the desired quantities.